1. Field of the Invention
The present invention relates to a drive control device for a display apparatus used in a video imaged is play apparatus such as a television picture reproducing device or a monitor for a computer, and more particularly to a drive control device for a display apparatus which realizes an ABL (automatic brightness limiter) or a function corresponding to the ABL.
2. Description of the Related Art
Some video image display apparatuses have an ABL (automatic brightness limiter) for limiting display brightness. Typically, the ABL conducts a control operation so that a mean display brightness of a screen is not too high for the purpose of suppressing power consumption or the like. A conventional CRT display apparatus is normally equipped with a CRT anode current detection type ABL circuit, which is made up of an analog circuit.
On the other hand, there is an ABL in a self-light-emission type flat panel display apparatus using a method that processes an analog video image signal that is input to a PDP, as described in JP 10-268832 A. Also, digitization of ABL control is disclosed in, for example, JP 2000-250463 A.
The conventional digital ABL control operation adjusts the brightness every one-frame scanning period.
Therefore, in the case where the setting resolution of a brightness control value which is a parameter for adjustment of brightness is coarse, a user feels visual annoyance from a change in the brightness of the overall screen caused by the ABL control in a video image having a mean brightness gently changed between a plurality of successive frames or a video image having the brightness of a part of a screen changed.
Under the above circumstances, in order to prevent the visual annoyance, a variation of the brightness control value has been intentionally reduced so as to smoothly change the brightness. However, in the video image whose mean brightness increases or decreases monotonously over a plurality of frames, a change in the brightness control value does not follow a change in the mean brightness. Therefore, even if there is no change in the mean brightness of an input video image, the brightness control value continues to change, with the result that the user feels strangeness.
This phenomenon will be described with reference to FIGS. 12A to 12C. FIG. 12A is a graph showing an example of the mean brightness of the input video images, which monotonously increases and is then stabilized. A brightness reference value, which is a desired upper limit of a display mean brightness is indicated by a dotted line and set in advance.
FIG. 12B is a graph showing a change of the brightness control value over time, which corresponds to the video image shown in FIG. 12A. A desired value of the brightness control value (a brightness suppression coefficient in this example) is indicated by a solid line, and in order to prevent visual annoyance, a variation of the brightness control value between the respective frames is suppressed so as to be small, and therefore the brightness suppression coefficient that is actually output is indicated by a broken line.
FIG. 12C is a graph showing a change in the mean brightness displayed as a result of operating the ABL over time. A portion where the solid line and the broken line are apart from each other in FIG. 12B is a time zone where the suppression of the brightness does not follow the change in the brightness, and after the mean brightness of the input image has been stabilized, the brightness that has exceeded the brightness reference value is gradually suppressed and then brought into agreement with the brightness reference value. It has been found that this is a factor that gives the user visual strangeness.